This invention relates to a stray-light suppressor for eliminating stray light, or the light of wavelengths other than of desired lengths which otherwise reaches the exit for outgoing light of a Littrow spectroscope.
Littrow spectroscope is an optical instrument for taking out an output of light of given wavelengths from the spectrum of a halogen lamp showing wide-band components.
The construction of an ordinary Littrow spectroscope is schematically shown in FIG. 1. Numeral 1 designates a light source, 2 a lens, 3 a slit, 4 a plane mirror, 5 a concave mirror, 6 a diffraction grating, 7 an exit, and 8 a beam of outgoing light.
Light from source 1 travels in the direction of the arrows in FIG. 1 toward exit 7. The full and broken lines indicate the width of optical path.
Light advances from source 1 in the following way:
(a) It is first reflected by plane mirror 4 toward concave mirror 5. PA1 (b) Reflected light from concave mirror 5 reaches diffraction grating 6. PA1 (c) Diffraction grating 6 reflects light back to concave mirror 5. PA1 (d) Re-reflected light from concave mirror 5, while being focused, reaches exit 7.
Since diffraction of light takes place on the surface of diffraction grating 6, rotation of the grating causes light beams of varying wavelengths to appear, in succession, at exit 7.
Depending on the angle of diffraction grating 6 so rotated, reflected light may shuttle between concave mirror 5 and diffraction grating 6 until part of it arrives at exit 7.
FIG. 2 illustrates the above arrangement producing stray light. As shown, light reflected twice by diffraction grating 6 and thrice by concave mirror 5 comes out as stray light 9. Stray light 9 is light of wavelengths other than desired lengths. It must be eliminated because, if allowed to reach exit 7, it can lead to an error in spectroscopic analysis.
FIG. 3 shows the spectrum of outgoing light 8 of FIG. 2. In FIG. 3 the primary light is the optical output required, and the secondary light represents higher harmonics of the primary light. The spectrum shows stray light emerging between the primary light and secondary light. The point where stray light appears shifts as diffraction grating 6 turns.
In order to eliminate stray light 9, as is obvious from FIG. 2, it is only necessary to reduce the frequency of light reflection between concave mirror 5 and diffraction grating 6 lest any reflected light other than the primary light find its way to exit 7. Reduction of reflection frequency might simply be achieved by extending plane mirror 4, concave mirror 5, and diffraction grating 6 laterally, or horizontally, as viewed in FIG. 2. However, this would pose a problem of increased size of the Littrow spectroscope.
Stray light might also be excluded by connecting exit 7 with a filter that passes only the light of desired wavelengths. However, it would call for too many filters to be practicable, because the outgoing light that results from the rotation of diffraction grating 6 involves light output of a wide band.